A typical exhaust after treatment system for an internal combustion engine may involve the placement of a catalyst treatment device in close proximity to the exhaust manifold of the internal combustion engine. This catalyst treatment device, referred to as a close-coupled catalytic converter, minimizes thermal loss in the exhaust gas, between the engine and the device, resulting in higher temperatures and quicker catalytic activation since the catalyst compounds that are typically used for treating engine exhaust gas operate best at temperatures in excess of 350° C.
Internal combustion engines that utilize a compressor such as an exhaust driven turbocharger to compress the combustion air charge, may be configured such that exhaust gas exiting the engine is conducted directly into, and through, the exhaust driven turbocharger. For greatest thermal efficiency, it may be desirable to locate a close-coupled catalytic converter directly adjacent to the outlet of the exhaust driven turbocharger in order to minimize the length of the exhaust gas passage therebetween; and resultant thermal load that must be overcome.
In such closely coupled arrangements, exhaust gas exiting the exhaust driven turbocharger during low speed operation may include a rotational or swirling component that migrates towards the outer circumference of the exhaust gas passage. As a result, upon reaching the inlet face of the catalyst substrate of the close coupled catalytic converter, the distribution of exhaust gas across the inlet face may be concentrated towards the outer circumference resulting in inefficient exhaust gas flow through the substrate. Such uneven flow of exhaust gas through the substrate may reduce the conversion efficiency of the exhaust treatment device.